Morbidity associated with malfunctions of the atria, while not immediate, is high. Atrial malfunctions of rhythmicity (e.g., atrial fibrillation, various atrial arrhythmias, A-V block and other conduction abnormalities, etc.) can contribute to thrombosis, emboli, stroke and/or heart failure, each of which can place a patient in significant peril.
Atrial Sensing. A variety of approaches have been developed which use pacemakers to counter atrial malfunctions of rhythmicity, as well as attendant effects on ventricular function. In addition, sophisticated approaches have been developed for pacemaker systems to determine the nature of any particular ventricular malfunction, and whether a malfunction originates in the atria or in the ventricles. One such approach uses ventricular sensing to measure/determine the probability density function (pdf) on a moment-to-moment basis. For example, U.S. Pat. No. 5,163,429 to Cohen discloses the use of narrow window pdf data as but one criterion among several for assessing ventricular cardiac function. The use of pdf data to determine ventricular fibrillation also is disclosed in Implantable Cardioverter-Defibrillators (N. A. Estes III, A. Manolis & P. Wang, ed.). U.S. Pat. No. 5,421,830 to Epstein, et al. (discussed further below) also discloses the use of pdf data as one set among a variety of data types that collectively are also used to assess cardiac function. The use of probability density function data for assessing atrial cardiac function has not been disclosed and presents its own unique difficulties as will be further discussed.
Electrical Test Stimulation of Atria. In a few limited cases, pacemaker protocols have been employed in which electrical test stimuli are applied to the atria, and the physiological responses thereto are monitored to aid in the determination of the best or most appropriate protocol to initiate, cure, or ameliorate the existing cardiac malfunction. For example, U.S. Pat. No. 5,620,471 to Duncan discloses three basic protocols for determining whether observed ventricular irregularities are actually caused by atrial arrhythmias. One protocol includes atrial electrical test stimulation, and all three protocols monitor both atrial and ventricular rhythms for three parameters: rates of atrial and ventricular firing, stability of firing/beating in atria and ventricles, and whether or not ventricular firing tracks atrial firing. In the first protocol, when the ventricular firing rate is less than the atrial firing rate (indicating no ventricular tracking of atrial beats), and firing rates are stable, then ventricular tachycardia is presumed, and ventricular stimulation is applied. On the other hand (second protocol), if the ventricular firing rate is not stable, then atrial arrhythmia is presumed, and atrial stimulation is applied. The third protocol is based on the fact that, when the ventricular firing rate equals the atrial firing rate, there may or may not be ventricular tracking of atrial firing. Whether or not there is ventricular tracking is determined by the presence or not of ventricular tracking following premature atrial stimulation by the pacemaker. If there is ventricular tracking of atrial firing, the arrhythmic mechanism is presumed to be atrial tachycardia. However, if there is no ventricular tracking of atrial firing, then ventricular tachycardia is presumed, and ventricular stimulation is performed.
U.S. Pat. No. 5,421,830 to Epstein, et al. discloses a general means for recording, testing, and analyzing cardiac function based on data from--and electrical test stimulation via--a patient's pacemaker, as well as data from additional sensors detecting hemodynamic or other body functions. Total intracardiac electrograms (reflecting both atrial and ventricular functional status) or just selected data (e.g., P-P or R-R intervals, heart rate, arrhythmia duration, slew rate, probability density function, etc.) may be recorded and analyzed. The patient's atrial and ventricular responses to electrical test pulses may also be recorded. In sum, this system provides a means to more easily tailor settings for pacemakers to achieve optimal settings for the specific patient or for the specific situation (e.g., during exercise or exertion) of a given patient.
U.S. Pat. No. 5,215,083 to Drane, et al. also discloses the use of electrical test stimulation to aid in the fine tuning and evaluation of different possible stimulation protocols for a patient's heart. In particular, electrical test pulses are employed to induce ventricular fibrillation or tachycardia for use in evaluating the effectiveness of alternative programmed therapies.
Multiple Site Atrial Stimulation. The use of multiple site atrial stimulation has been disclosed for various purposes, such as defibrillation, cardioversion, pacing, and dc field production. One example is provided by U.S. Pat. No. 5,562,708 to Combs, et al., which discloses the employment of large surface electrodes (each effectively comprising multiple electrodes) that are implanted to one or both atria for providing extended, low energy electrical impulses. The electrical impulses are applied simultaneously at multiple sites over atrial surfaces, and atrial fibrillation is interrupted by gradually entraining greater portions of atrial tissue. These pacemaker electrodes may be used for various purposes in addition to pacing, such as conventional defibrillation and cardioversion.
U.S. Pat. No. 5,649,966 to Noren, et al. discloses the use of multiple electrodes for the purpose of applying a subthreshold dc field to overcome fibrillation. The rate of application of the dc field is sufficiently low so that no action potential is triggered. Polarity may also be changed periodically. In one embodiment, four electrodes are positioned within a single plane in the heart, which permits a dipole field in virtually any direction within that plane.
U.S. Pat. No. 5,411,547 to Causey, III discloses the use of sets of complex mesh patch electrodes, in which each electrode comprises an anode patch and a cathode patch, for purposes of cardioversion-defibrillation. Bidirectional cardiac shocking is permitted by these electrodes.
U.S. Pat. No. 5,391,185 to Kroll discloses the use of multiple electrodes to effect atrial defibrillation. The possibility of inducing ventricular fibrillation during the course of atrial defibrillation is greatly reduced by synchronizing the atrial stimulation to fall within the QRS phase of the ventricular cycle.
U.S. Pat. No. 5,181,511 to Nickolls, et al. discloses the use of multiple electrodes in antitachycardia pacing therapy. The electrodes not only each serve an electrical sensing role (to locate the site of an ectopic focus), but also function in concert to create a virtual electrode for stimulating at the site of an ectopic focus.
Existing Needs. In the area of atrial malfunctions of rhythmicity what is needed is a means to entrain multiple atrial sites, but also in combination with an atrial sensing/measurement capability that is coupled with atrial test stimulation and analysis capability. Atrial test stimulation and analysis capability is needed to provide better determination of the nature of the malfunction and the most probable or efficacious corrective therapy to undertake. Furthermore, the use of atrial test stimulation is critically needed for the fundamental reason that the physician cannot know a priori how a given heart (or a given heart under a particular medical or pathological condition) will respond to a selected stimulation regime, even if that selected stimulation regime would work generally for other cardiac patients. Thus, a trial-and-error testing capability needs to be available for pacemakers whose traditional stimulation regimes do not work for the occasional refractory patient. The multiple site stimulation capability is needed in order to more quickly and efficiently cardioconvert the atria in the face of arrhythmia, fibrillation, etc. Atrial sensing and use of measurement data are needed to better provide the physician and/or the circuit logic of the pacemaker with information as to the physiological state of the heart; i.e., whether there is atrial arrhythmia or fibrillation, where an ectopic focus is located, etc. Thus, what is needed is a pacemaker that combines all three of these elements: atrial sensing and measurement capability, atrial electrical test stimulation and analysis capability, and multiple site stimulation capability.
Lastly, a need also exists for a stimulation protocol which can travel more quickly across the myocardium and which provides improved cardiac entrainment along with the ability to entrain portions of the heart from a greater distance.